Question: Which of these numbers is prime? ${17,\ 25,\ 45,\ 51,\ 58}$
A prime number has exactly two factors: 1 and itself. The factors of 17 are 1 and 17. The factors of 25 are 1, 5, and 25. The factors of 45 are 1, 3, 5, 9, 15, and 45. The factors of 51 are 1, 3, 17, and 51. The factors of 58 are 1, 2, 29, and 58. Thus, 17 is a prime number.